Will an advanced degree make you more marketable? The answer isn't a simple one. It depends on your career goals. Some occupations require a master's degree or higher for entry-level jobs. For example you need at least a master's degree if you want to be a librarian, speech pathologist, archaeologist or physical therapist and a doctoral degree if you want to work as a clinical or counseling psychologist. You need a graduate degree to advance in many other occupations. For example, a registered nurse doesn't need a master's degree to get a job, but if he or she wants to move into a managerial position, a graduate degree can help with that.
When the career to which you aspire requires an advanced degree, the answer to the question "Should I apply to grad school?" is pretty straightforward. It's much less so if you need the degree in order to advance in your career. First you have to decide how far up the ladder you want to climb. Then you need to figure out when to begin pursuing that goal. If you know you want to go to grad school eventually and are having trouble finding a job, you may decide that this is a good time to do it. You'll have lots of competition though. Many people decide to return to school when the market is bad.